Light-induced heat and mass transfer in a single-component gas in a capillary
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Light-Induced Heat and Mass Transfer in a Single-Component Gas in a Capillary I. V. Chermyaninov, V. G. Chernyak, and E. A. Vilisova Ural State University, Yekaterinburg, 620083 Russia e-mail: [email protected] Received February 14, 2007
Abstract—A theoretical analysis is presented of light-induced heat and mass transfer in a single-component gas in a capillary tube at arbitrary Knudsen numbers. Surface and collisional mechanisms of transfer are analyzed, due to differences in accommodation coefficient and collision cross section between excited- and ground-state particles, respectively. Analytical expressions for kinetic coefficients characterizing the gas drift and heat transfer in a capillary tube are obtained in the limits of low and high Knudsen numbers. Numerical computations are performed for intermediate Knudsen numbers. Both drift and heat fluxes are determined as functions of the light beam frequency. In the case of an inhomogeneously broadened absorption line, the lightinduced fluxes are found to depend not only on the sign, but also on the amount, of light beam detuning from the absorption line center frequency. PACS numbers: 51.10.+y DOI: 10.1134/S1063776107090063
1. INTRODUCTION Light-induced drift (LID) of particles in a buffer gas caused by velocity-selective excitation by monochromatic light was predicted in [1, 2]. In the surface mechanism of LID (surface LID), the role of a buffer gas is played by the wall of a channel, due to dependence of particle–surface interaction on the particle’s state. Surface LID was studied experimentally and theoretically in [3–11]. In [12, 13], a collisional mechanism of LID was predicted for a single-component gas near a wall. By virtue of momentum conservation, this LID mechanism cannot take place in a bulk gas. However, a difference in collision cross section between excited- and groundstate particles implies a difference in Knudsen layer thickness (which is proportional to mean free path). Indeed, since the collision cross section is larger for excited-state particles as compared to ground-state particles, they have a shorter mean free path. Because the corresponding Knudsen layer thickness is smaller, the mass flux of excited-state particles cannot be balanced by the opposite flux of ground-state particles. The result is an overall gas flux along the wall parallel to the mass flux of ground-state particles. Studies of LID of single-component gases in capillaries were reported in [4, 5, 14, 15]. Surface LID under slip-flow and free-molecular flow conditions was analyzed in [4] and [5], respectively. In [14, 15], the surface and collisional mechanisms of LID of a singlecomponent gas in a capillary tube were modeled for arbitrary Knudsen numbers Kn (defined as the ratio of the molecular mean free path to the tube radius). The
calculations were based on the second-order approximation for the collision operator in the McCormack model kinetic equations [16], which provide a satisfactory description of either mass or heat flux in a gas. Simultan
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